Fused salt electrolysis to obtain manganese metal

ABSTRACT

THIS INVENTION PROVIDES A METHOD FOR OBTAINING MOLTEN MANGANESE METAL BY ELECTRLYSING A MOLTEN MIXTURE OF METAL HALIDES COMPRRISING A MANGANESE HALIDE AND A BATH MIXTURE COMPRISING A HALIDE OF A REACTANT METAL, AN ALKALI METAL HALIDE AND AN ALKALINE EARTH METAL HALIDE. MANGANESE METAL IS OBTAINED IN A MOLTEN PHASE BELOW THE MOLTEN MIXTURE OF METAL HALIDES AND ELEMENTAL HALOGEN IS EVOLVED AT THE ANODE. THE REACTANT METAL IS ELECTRLYTICALLY REDUCED AT THE CATHODE. THE ANODE AND CATHODE ARE BOTH INERT AND IMMERSED IN THE MOLTEN MIXTURE OF METAL HALIDES THE HALIDES ARE PREFERABLY CHLORIDES, BROMIDES OR   IODIDES. THE REACTANT METAL CAN BE ANY CATHODICALLY REDUCIBLE METAL WHICH CAN REPLACE MANANESE FROM MANGANESE HALIDE IN THE MOLTEN SALT BATH, BUT IS PREFERABLY ALUMINUM OR MAGNESIUM.

B. E, BARTON EVAL FUSED SALT ELECTROLYSIS To OBTAIN MANGANESE METALFiled Nov. 27, 1972 Aug. 27, 1914 3 2 L W .T /w//Nv/////////f/////////f/r/r//// 2 H v 6 UNM HH/ 2 HH 1 //W/ //,.H, f/// ,////L i////lNvl-:N'rons PAUL H. CARDWELL BRUCE 3E. BARTON United States PatentOffice 3,832,295 Patented Aug. 27, 1974 U.S. Cl. 204-64 R 12 ClaimsABSTRACT OF THE DISCLOSURE This invention provides a method forobtaining molten manganese metal by electrolysing a molten mixture ofmetal halides comprising a manganese halide and a bath mixturecomprising a halide of a reactant metal, an alkali metal halide and analkaline earth metal halide. Manganese metal is obtained in a moltenphase below the molten mixture of metal halides and elemental halogen isevolved at the anode. The reactant metal is electrolytically reduced atthe cathode. The anode and cathode are both inert and immersed in themolten mixture of metal halides.

The halides are preferably chlorides, bromides or iodides. The reactantmetal can be any cathodically reducible metal which can replacemanganese from manganese halide in the molten salt bath, but ispreferably aluminum or magnesium.

Many non-ferrous metals are commercially produced at relatively lowcost; however, although the metal is suitable for certain purposes, itis too impure for other possible uses. It thus often becomes necessaryto go to substantial additional expense to purify the metal, which wasobtained, for example, by reduction of compounds found in the ores.Manganese, for example, has some uses which are satisfied by arelatively low grade purity metal, e.g., in which the manganese cancontain carbon silicon or iron. For other purposes, however, anextremely high purity material is required in which the presence of anycarbon, silicon, or iron would `be detrimental. For other uses, onlycertain of the impurities impose a problem and the presence of otherimpurities is not detrimental. For example, manganese is often found iniron or steel products. For such uses the presence of even a relativelysmall percentage of carbon would be deleterious as is could have asignificant effect upon the properties of the steel in which themanganese is present. The presence of a small proportion of iron in themanganese would have substantially little or no effect on the finalproperties of such a product.

Another factor is the form of the metal obtained from the process. Forexample, there is often a requirement that the metal be in special form,eg., in large chunks or ingots.

Manganese, for example, has been prepared by several methods; generally,all of these methods comprise reducing a manganese oxide to theelemental metal. Such reduction has been carried out by displacing themanganese from the oxide using aluminum metal, by reducing using carbon,or by electrolysis such as in a fused salt bath. The reduction of themanganese oxide by contact at high temperatures with carbon or aluminumdoes result in a relatively large size chunk of the metal, butunfortunately, it does not result in a high purity product.

The electrolysis of oxides of manganese in a fused salt bath has beencarried out and this does result in large chunks or ingots of metal ofrelatively good purity, if great care is taken. The procedures whereinmanganese oxide is reduced have been extremely expensive, however.

Various fused salt electrolysis methods have been carried out, whereinthe manganese feed has been manganese oxide ores, mixtures offerro-manganese and calcium fluoride and calcium manganite. These,however, were generally unsuccessful in preparing high purity manganesemetal products.

An early worker in the field, Mitchell, in U.S. Pat. No. 2,398,589,suggested the electrolysis of manganese oxides in a fused bathcontaining an alkali metal fluoride plus an aluminum or magnesium oxide.For example, the bath comprised sodium fluoride, aluminum oxide ormagnesium oxide, and a manganese oxide, preferably MnO. In order toa'void the problem of additional impurities being dissolved from thecontainer material, the sides of the cell were lined with manganeseoxide. Manganese metal was formed as a solid mass on the bottom of thecell. A metal cathode was located at the bottom of the cell and a carbonanode was located at the top of the cell. The current liow was in avertical direction.

The problem of removing a solid mass of manganese from the bottom of thefused bath cell was difficult and subsequent workers, such as Welsh etal., in work described in U.S. Pat. No. 3,018,233, devised methods toobtain molten manganese metal which could then be removed by tapping thecell at regular intervals. Welsh et al. disclosed that an oxide ofboron, aluminum or silicon can be added to the manganese oxide-calciumfluoride salt to increase the solubility of the manganese oxide in theelectrolyte. The boron oxide, for example, is added in almost equalproportions with the manganese oxide. Welsh et al. formed a molten layerof manganese at the bottom of the cell which then acted as the cathode.A carbon anode was used projecting downwardly from the top of the cell.The current ow was in a vertical direction through the fused salt mass.

Although Welsh et al. was an improvement over Mitchell, the same problemarose with regard to the presence of oxides which generally make itsubstantially impossible to avoid at least some contamination of themetal by the high melting, high boiling oxide materials.

It has now been discovered that it is possible in accordance with thepresent invention to obtain substantial pure molten manganese in a cellutilizing inert anodes and cathodes in which the molten electrolyte iscomprised only of halides. The product from the cell of this inventionis a pure manganese metal in which the presence of oxides issubstantially eliminated and from which substantially all of thereaction products formed during the electrolysis are removed byIvaporization, resulting in the production of manganese metal which issubstantially pure.

In accordance with the present invention, a process for formingsubstantially pure molten manganese metal is provided. The processcomprises first forming a molten mixture of halides, comprising amanganese halide, a bath halide selected from the group consisting ofalkali metal halides and alkaline earth metals halides, and a halide ofa reactant metal. The rectant metal can be any cathodically reduciblemetal which can replace manganese from its halide, but is preferablymagnesium or aluminum.

The mixed fused halide bath is electrolyzed to form manganese metalwhich sinks through the bath and collects as a molten layer at thebottom of the bath while halogen is collected at the anode. Heat isapplied to the bottom of the bath to maintain the manganese metal in amolten state while the top of the bath is preferably maintained at asufficiently low temperature to prevent the vaporization and loss ofmetal halides, but at least about 650 C. A temperature gradient can bemaintained through the molten bath to avoid vaporization of any halides.

The apparatus in which the electrolysis procedure is carried out is afurnace wherein the upper portion of the furnace is a fused saltelectrolytic cell and the lower portion is a molten metal collectionsection. The temperature maintained in the lower metal collectionportion of the furnace is sufficient to maintain the manganese metal inthe molten condition, but the temperature in the upper cell portion ofthe furnace is sufficient to maintain the metal halides in a moltencondition, but is below the boiling point of the metal halide mixture.The electrodes are located in the upper, or cell portion, of the furnaceand the ow of electricity is in a horizontal direction across the bath.

It is believed that the present process does not result in the directcathodic reduction of the manganese halide to manganese metal. Rather,it is believed that the halide of the reactant metal, e.g., magnesiumhalide or aluminum halide, is cathodically reduced to form the halogenand the reactant metal. The, e.g., magnesium metal or aluminum metalwhich is formed then reacts with the manganese halide present in thebath, replacing the manganese in the halide salt to form elementalmanganese metal and the eg. aluminum halide or magnesium halide. Theelemental manganese metal then sinks to the bottom of the halide bath toform a layer of molten manganese metal.

The potential drop between the electrodes is not sufcient tocathodically reduce the manganese halide to manganese metal. It isbelieved that the potential drop is sufficient, under the conditions ofthe process of this invention, to cathodically reduce the halide of thereactant metal to the elemental reactant metal.

Preferably, the temperature of the halide bath is above the meltingpoint of the reactant metal, e.g. 650 C. for magnesium and 660 C. foraluminum.

Preferably, the reactant metal is formed as small particles or dropletsdispersed in the molten mixed metal halide bath. This provides a greatersurface area for reaction between the reactant metal and the maganesehalide. To further prevent the agglomeration of the molten reactantmetal, an anti-agglomerating agent is added, which serves to maintainthe molten reactant metal in a dispersed state in the molten halidebath. This is especially useful when magnesium halide is present.

The bath halide and the halide of the reactant metal are miscible withmanganese halide but immiscible with manganese metal. The bath halidesare generally inert to the manganese halide under the conditions of thisprocess and do not react with the cathodically reduced reactant metal,e.g. magnesium or aluminum. They are essentially inert with regard tomanganese metal and generally, are non-corrosive to the usual types ofhigh temperature ceramic liners available.

The bath halides must be a mixture of at least one alkali metal halideand at least one alkaline earth metal halide. The mixture can form a lowmelting eutectic composition so as to reduce the temperature, at leastin the upper electrolysis cell portion of the apparatus.

Certain of the halides used, especially manganese halides and aluminumhalides (if used as reactant halides) have relatively high vaporpressures under the conditions of the electrolysis cell. Aluminumhalides, in the pure state, boil at temperatures substantially lowerthan those under which these cells are normally operated, e.g. about 650C. and higher. However, when the aluminum halides are admixed with thealkali and/or alkaline earth metal halides of the bath halide, the vaporpressure of the mixture is substantially reduced, apparently by theformation of certain complexes, and the boiling point of the mixture atatmospheric pressure is raised to substantially above 650 C.

Preferably, there is present at least one alkali metal halide. Usefulalkali metal halides comprise the chlorides, iodides, and bromides ofsodium, potassium, cesium, rubidium and lithium. |Useful alkaline earthmetal halides comprise the chlorides, bromides, and iodides of calcium,barium, strontium, and magnesium.

In determining composition of the molten halide mixture, the bathmixture, refers to the combined reactant halide and bath halide, i.e.everything except the manganese halides.

`The bath mixture comprises from about 30 to about 60% by wt. of thebath mixture of an alkali metal halide, about 10 to about 25% by wt. ofa halide of a reactant metal and from about 15 to about 60% by wt. of analkaline earth metal halide. It is understood by the above stated rangesof composition, that when, e.g. a magnesium halide is utilized as thehalide of a reactant metal, the total proportion of magnesium halidepresent in the bath mixture can be from about 10 to about 70% by Wt. ofthe bath mixture.

In addition, there is preferably also present in the bath mixture asmall proportion of an anti-agglomerating agent, such as an alkali metaltetraborate; generally, not more than 0.05% by wt. of the compound andpreferably from 0.001 to about 0.01% by wt.

There can be present from about 0.2 to about by wt. of the total mixtureof a manganese halide. Preferably, however, the manganese halide ispresent in an amount of from about 0.5 to about 30% by wt., andoptimally from 0.5 to about 10% by wt.

The temperature of the salt bath should be just sufficient to maintainthe halide and preferably also the reactant metal in the moltencondition. It is desirable to avoid loss by vaporization of any of thevolatile halides. This is especially true of the aluminum halide ifused.

Accordingly, the temperature of the top surface of the molten mixedhalide bath can vary from 650 C. up to about 1260 C. When aluminumhalide is used as the reactant halide, at higher temperatures, thevolatile halide can be lost by vaporization. Accordingly, the preferredtemperature for the top surface of the mixed halide bath containingaluminum halide is from about 650 C. to about 1,000 C. and preferablyabout 800 C. to about 1,000 C. The molten manganese layer at the bottomof the bath is preferably maintained in the range of from about l,260 C.to about 1,300 C.

Useful anti-agglomerating agents includes boric oxide or metal salts orboric acids for example B203, Na2B4O7 and K2B2O4 The drawing is aVertical section of a proposed combination fused salt electrolytic cellfurnace.

Referring to the drawing, the fused salt electrolytic cell furnaceincludes an insulating brick outer shell 10, a refractory brick lid 12,and a high temperature-resistant inert liner 32, such as high purityalumina in brick or solid sheet form, or high purity Zirconiain brick orsolid f'sheet form. The liner is supported by a metal backing 34;

graphite anodes, 24 and 28, extend through the top of lid 12 into thecell and are connected, via conductors 20 and 21, to the anode bus of anelectrical power source, not shown. A steel cathode, 26, extends intothe cell and is connected, via electrical conductors 22, to the cathodebus of an electrical power source, not shown. Solid feed inlet ports 16and 17, are provided extending through the refractory lid 12, and vaporoutlet ports 14 and 18 are also provided through refractory lid 12.

The lower portion of the furnace is formed as a relatively narrow neck38 from which a liquid outlet port 40 is provided. The lower narrowportion 38 is coaxial with the larger portion 42 of the furnace and issurrounded by heating chamber 36. A bleed line 23 is provided from theupper portion 42 of the furnace.

Heating elements are positioned within the heating chamber 36. Anyconventional heating elements can be provided in chamber 36. Moltenmanganese is recovered through tap 40.

The anodes 24 and 28 are preferably formed of graphite. The cathode 26is preferably formed of steel. The electrodes must be inert to the fusedsalt medium, the halogen and the manganese metal and reactant metal. Thecathode must be formed of a material having a sufficiently high meltingpoint that it is not softened or weakened during the electrolysis whenimmersed in the fused molten salt bath.

The outlet port for the molten metal and the bleed line for the moltenmetal halides are conventional in the art. Basically, such ports includean outlet which can be stopped by a removable clog of refractorymaterial. The molten material can be removed continuously orintermittently as -desired in a continuous process. Designs of suchoutlets are `well known in the art. See, for example, U.S. Pats. Nos.'3,018,233 and 2,309,598.

Any heat source can be used in chamber 36. Gas jets and electrical coilsare useful, or alternatively, heating coils can be positioned within thereactor vessel portion, 38, or heating electrode plates can be providedflush within the walls 32, of the lower portion, 38.

In operation,v it is extremely important to prevent the presence ofoxygen or any oxides in the bath, except for the antiagglomerationagent. Metal oxides tend to increase the amount ofjimpurities in themanganese metal. Accordingly, before adding any manganese halide to thecell, the cell is preferably ushed with an inert oxygen-free gas, suchas nitrogen or any other inert material, to remove all air, after thebath halides have been added to the cell and heated to a temperaturesufficient to melt all of the halides'including the alkali metal,alkaline earth metal halides and any other reactant metal halidepresent. The lower portion of the melt in the narrow neck portion 38 ispreferably maintained at a temperature of at least about 1260" C. andcan be kept as high as 1300 C.

When the temperature of the bath has been set, the electrodes areenergized to the desired voltage dependent upon the reactant metal used.Manganese halide is added through solid inlet ports, 16, and 17. Themanganese halide melts as it mixes with the fused salt bath.

Halogen is evolved at the anodes and can be withdrawn through ports 14and 18.

Gradually, a `layer of molten manganese metal is collected in the narrowneck portion 38. This molten manganese metal can be intermittentlyremoved through tap 40.

Generally, the manganese halide that is added to the cell will not becompletely pure. Manganese halide which is obtained, for example, fromthe refining of ocean oor nodule ore, such as for example, is describedin German Patent Specification No. 2,126,175 contains substantialproportions of sodium halide, magnesium halide, potassium halide,lithium halide, and calcium halide. Thus, the bath is being fed not onlywith the manganese halide, which is being reduced and removed, but alsoadditional alkali and alkaline earth metal halides. In order to preventa buildup of the halides in the cell and to maintain a substantiallyconstant cell bath concentration, a continuous or intermittent bleed ismaintained through exit port 23 from the electrolysis cell. The rate ofbleed through line 23l must be balanced against the rate of feed of theimpure, manganese halide and the composition of the feed material inorder to maintain a substantially constant electrolyte concentration.

As stated above, it is believed that the present procedure does notresult in the direct electrolysis of manganese. The electrolysis occursas the cathodic reduction of the reactant halide, e g. magnesium halideor aluminum halide, to form e.g. magnesium metal or aluminum metal andthe elemental halogen. The halogen is removed at the anode. Under theconditions of the cell, all of the halogens, i.e., chlorine, iodine orbromine are in the vaporous state and can be readily removed. Thereactant metal, eg. aluminum 0r magnesium which is formed remains in thesalt bath and reacts with the manganese halide, displacing the manganeseto reform mangesium halide or aluminum halide and the elementalmanganese metal. The elemental manganese metal sinks through the moltenhalide bath to collect as a lower layer in the narrow neck, or furnace,portion 38, of the cell-furnace combination. Thus, there issubstantially no net loss of the reactant halide, e.g., magnesium halideor aluminum halide, from the cell.

The manganese halide must be replaced in the bath during electrolysis.The rate at which the manganese halide is consumed is dependent upon therate at which the reactant metal is formed, which in turn is dependentupon the current flow. The rate of feed of the manganese halide must besufcient to maintain a suicient concentration of manganese halide in theelectrolysis bath to react with the reactant metal to form manganesemetal and the halide of the reactant metal.

The following examples provide preferred embodiments of the presentinvention, but are exemplary and not exclusive of the scope thereof:

EXAM PLE 1 A sample of mixed metal chlorides was prepared having thefollowing composition by weight:

Components: -Percent by Weight Sodium Chloride 59.8 Magnesium Chloride24.9 Calcium Chloride 15.0

Sodium Tetraborate 0.3

A 10,000 gram sample of the above mixture was added to an alumina-linedelectrolyte cell, of the type shown in the drawing, and melted to form afused molten salt bath. The temperature in the lower portion 38 of thefurnace ir the drawing, was 1260 C. The temperature of the upper surfaceof the bath was 840 C.

The electrodes were energized and a potential drop of 6.4 volts wasformed between the electrodes at a current density of 500 amps/sq. ft.After 15 minutes of operation, anhydrous manganese chloride 58 grams wasadded. The test was continued until 300 ampere-hours of current hadpassed and a total of 580 grams of anhydrous manganese chloride had beenadded. The voltage drop between the electrodes had decreased from 6.4 to5.0 during this period and the temperature of the upper portion of thebath had increased to 895 C.

Manganese metal was produced during the test and collected in the lowernarrow portion 38 of the cell. The netal was periodically tapped andremoved through port The current efficiency of the electrolysis basedupon the amount of manganese collected was percent. Chlorine was evolvedand removed through the ports 14 and 18.

EXAMPLE 2 A mixture of metal halides having the following composition isprepared Components: Percent by Weight Sodium Chloride 40 AluminumChloride 20 Calcium Chloride 40 A sample of 10,000 grams of the abovesalt mixture is melted as described in Example 1 in the combinedelectrolysis cell furnace, shown in the drawing. The temperature of theupper portion of the melt is maintained at 860 C. The temperature at thevery bottom portion 38 is maintained at 1260o C. The electrodes areenergized and a potential drop of 6.3 volts and a current density of 500amp/sq. ft. is applied between the electrodes. After the current hasbeen applied for 15 minutes, 58 grams of anhydrous manganese chloride isadded to the bath. The test is continued until a total of 300 amperehours of current has been passed and 580 grams of manganese chloride hasbeen added to the electrolysis cell.

Manganese metal was collected in the bottom portion 38 of the cellfurnace. Manganese metal was intermittently tapped and removed throughtap 40.

The patentable embodiments of this invention which are claimed are asfollows:

1. A process for the forming of substantially pure molten manganesemetal from manganese halide, the process comprising: (l) forming amolten mixture of metal halides, the halides being selected from thegroup consisting of chlorides, bromides and iodides, the molten mixturecomprising a manganese halide and a bath mixture comprising a halide ofa reactant metal, an alkali metal halide and an alkaline earth metalhalide; (2) passing an electric current through said molten mixturebetween an inert anode and an inert cathode immersed within the moltenmixture of halides; (3) electrolytically reducing the reactant metal atthe cathode; (4) evolving at and collecting from the anode an elementalhalogen, and collecting molten manganese metal below the molten mixtureof halides.

2. The process of Claim l wherein the reactant metal is selected fromthe group consisting of aluminum and magnesium.

3. The process of Claim 2 wherein the temperature of the lower portionof the bath immediately adjacent to the manganese metal is maintained atleast at the melting point of manganese metal, and the temperature ofthe upper portion of the bath is maintained at least at the meltingpoint of the mixed halides.

4. The process of Claim 2 wherein the reactant metal is aluminum.

5. The process of Claim 2 wherein the composition of the bath mixturecomprises from about 30 to about `60 percent by weight of an alkalimetal halide, from about 10 to about 25 percent by weight of a halide ofa reactant metal and from about l5 to about 60 percent by weight of analkaline earth metal halide.

6. The process of Claim 2 wherein the reactant metal is magnesium.

7. The process of Claim 3 wherein the temperature of the top portion ofthe molten metal halide bath is at least about 660 C.

8. The process of Claim 2 wherein the halide is chloride and the halogenis chlorine.

9. The process of Claim 5 wherein the bath mixture comprises sodiumchloride, calcium chloride, and magnesium chloride.

UNITED STATES PATENTS 3,450,524 6/1969 Pascaud et al. 204-64 R 3,535,21510/1970 Winand 204-64 R JOHN H. MACK, Primary Examiner D. R. VALENTINE,Assistant Examiner U.S. C1. X.R.

